The term "magnetron sputter" denotes an industrially well established technology for PVD coating of objects (substrates) by sputtering of solids ("targets"). This sputtering of the target is accomplished by bombardment with high-energy ions, usually ions of the rare gas argon (sputter gas). The ionic bombardment is initiated by gas discharge processes. The ions of the sputter gas are subjected to controlled acceleration in the direction of the target at such high kinetic energies that individual atoms and even crystallites are knocked out of the target. The process may be carried out either in direct current mode (DC magnetron sputter) or in alternating current mode (AC magnetron sputter). U.S. Pat. No. 4,166,018 discloses well-defined array of permanent magnets (here called `outer magnetron` by way of distinction from a term `inner magnetron` to be introduced later on) immediately behind the target distorts the trajectory of the electrons likewise present in the plasma and so achieves an appreciable enhancement of the gas plasma density in front of the target, thereby significantly increasing the sputter rate compared to ionic bombardment without the use of permanent magnets. A preferred type of magnetron sputter is unbalanced magnetron sputter. Here, the relative range of the magnetic field lines is prolonged far into the interior of the coating chamber by way of annular coils usually mounted outside of the coating chamber. The magnetic field can also be modified by control of the coil current.
To ensure as long a mean free path length of the mostly neutral target vapor particles (sputter particles), the entire magnetron sputter process is carried out in a closed coating chamber under conditions of high vacuum at an internal pressure of about 10.sup.-3 to 10.sup.-6 mbar. Some of the sputter particles deposit on the surface of the substrates, likewise arranged in the coating chamber, and envelope them in the course of coating with a thin layer. By applying a negative electric potential ("bias") to the substrate, or in the case of electrically non-conductive substrates, to a bias cathode in the neighborhood of the substrate (generally components of the substrate holder system), the layer morphology can be additionally influenced in a controlled manner. In typical industrial applications, the thin layer has a mean thickness in the range from about 0.1 to 5 microns. By a mixture of one or more reactive gases, usually nitrogen, oxygen or a gas containing carbon, into the coating chamber, chemical reactions or incorporations of the reactive gas atoms in the atomic structure of the thin layers, controlled modifications of the properties of the coating, such as hardness or adhesiveness, can be generated as well. With use of several different target materials, in the form either of one mixed target or in the form of at least two pure targets, multi-layer coatings can additionally affect the properties of the entire layer usefully. Multilayer coatings are usually produced by well-defined, alternating `switching off and on` of individual targets. The improvement of quality and economy in multilayer coatings is the chief concern of the invention.
Such multilayer PVD coatings are desirable wherever the individual layers may assume different functions. For example, with a two-layer coating, it is possible to produce good strength of adhesion to the substrate material by means of a first layer, and to generate the surface functionality actually desired, for example tribological behavior or decorative effects, by means of an overlying second layer.
Devices for practicing multilayer magnetron sputter are known from a number of publications and patents. The devices and practices related to the invention differ primarily in the layouts of the instrumental target-cathode-magnetron subsystem. Thus, U.S. Pat. No. 4,166,018 describes what can be referred to as a planar magnetron. Here, the target, rectangular in shape and preferably serving as cathode at the same time, is mounted with cooling directly over a set of permanent magnets. The magnetic field substantially increases the density of ionization of the sputter gas in the neighborhood of the target, thereby positively influencing the rate of attrition. A disadvantage is the highly non-homogeneous removal of material, manifesting itself in the form of erosion pits on the target.
To improve the removal of material U.S. Pat. No. 4,356,073 proposes a hollow cylinder target geometry including permanent magnet means. By gradually rotating the target about its axis of rotation, the non-homogeneity of removal can be largely compensated. This patent also disclosed how azimuthally segmented differences in composition of the material of the hollow cylinder target and rotation of the target can be used to deposit different materials on a substrate in the form of multilayer coatings. Besides the complicated process of production and assembly of such azimuthally segmented hollow cylinder targets, a disadvantageous effect is that a simultaneous and undesirable sputtering of different segments is difficult to avoid, so that an unintentional mixed coating of the substrate results.
In place of an azimuthally segmented hollow cylinder target, U.S. Pat. No. 4,443,318 proposes a target system made up of numerous individual planar target strips. The cylindrical cross-section is thus approximated by a polygonal cross-section, and the individual target strips are more readily replaced after wear than in the case of the azimuthally segmented hollow cylinder targets. However, by this modification, the improved utilization of the target material, advantageous for the hollow cylinder target according to U.S. Pat. No. 4,356,073, is sacrificed.
A disadvantage of all known processes for multilayer PVD coating of substrates by means of a magnetron sputter system is that such a coating essentially entails the following problems. Either individual targets must be alternately excluded, usually by disconnecting and covering with a so-called "shutter," from the coating process, thereby reducing the overall productivity of a system, or the coating process must be interrupted, and the next layer vapor deposited only after a mechanical change of target. A change of target always involves, in addition to the mechanical manipulation of units, a time-consuming process of prior aeration and subsequent evacuation. Further, in the case of a mechanical change of target, there is always a great danger that undesirable contamination will reach the substrate surface already produced, with adverse consequences to the quality of the coating as a whole. To avoid target changes, there is a multichamber system can be employed. However, this involves uneconomical, operating and maintenance costs. Such multichamber sputter systems are not competitive for purposes of small and medium mass production.
EP 0,495,215 and GB 1,194,428 describe evaporating devices having so-called turnabout targets. Target plates to be subjected to vapor deposition are mounted at the ends of a cross holder, having for example four axes. By rotating the cross holder, a different target plate can be brought into working position with respect to the coating chamber in each instance. A disadvantage of both of these patents is that no magnetic means are provided for generating the magnetron effect.